FIG. 1 is a circuit diagram illustrating a conventional current resonance type power supply device. In FIG. 1, a smoothing capacitor C1 is connected across output ends of a full-wave rectifier circuit RC1 which rectifies an AC (alternating current) voltage, and a series circuit of a switch element Q1 and a switch element Q2 each constructed of a MOSFET (metal oxide semiconductor field effect transistor) is connected across the output ends of the full-wave rectifier circuit RC1. The switch element Q1 and the switch element Q2 are alternately turned on and off. A series circuit of a resonance reactor Lr, a primary winding P of a transformer T and a current resonance capacitor C2 is connected across ends of the switch element Q2.
A secondary winding S1 and a secondary winding S2 of the transformer T are connected in series, an anode of a diode D1 is connected to one end of the secondary winding S1, and an anode of a diode D2 is connected to one end of the secondary winding S2. A cathode of the diode D1 and a cathode of the diode D2 are connected to one end of a smoothing capacitor C3, and the other end of the smoothing capacitor C3 is connected to a junction of the one end of the secondary winding S1 and the one end of the secondary winding S2. A detector 11 is connected to both ends of the smoothing capacitor C3. Incidentally, a leakage inductance of the transformer T may be used in place of the resonance reactor Lr.
The detector 11 detects an output voltage from the smoothing capacitor C3 and outputs the output voltage to an oscillator 13. The oscillator 13 generates a frequency signal obtained by varying an oscillation frequency according to the output voltage from the smoothing capacitor C3. A comparator CM1 outputs a high level if the frequency signal from the oscillator 13 is equal to or more than a divided voltage obtained by dividing a voltage of a power supply Vcc by a resistor R1 and a resistor R2, or the comparator CM1 outputs a low level if the frequency signal from the oscillator 13 is less than the divided voltage obtained by dividing the voltage of the power supply Vcc by the resistor R1 and the resistor R2.
An inverter IN1 inverts an output from the comparator CM1 and uses the inverted output to turn on or off the switch element Q2. A high-side driver 12 uses the output from the comparator CM1 to turn on or off the switch element Q1.
Next, description will be given with regard to operation of the conventional current resonance type power supply device. First, when the switch element Q1 is turned on, an electric current flows through a route from the full-wave rectifier circuit RC1 through the switch element Q1, the resonance reactor Lr, the primary winding P, and the current resonance capacitor C2 to the full-wave rectifier circuit RC1. The electric current is a combined current of an excitation current flowing through an excitation inductance Lp on a primary side of the transformer T, and a load current supplied from an output terminal OUT to a load through the primary winding P, the secondary winding S2, the diode D2, and the capacitor C3. The former current is a resonance current in the form of a sinusoidal wave of (the reactor Lr plus the excitation inductance Lp) and the current resonance capacitor C2, and is observed as a current in which a portion of the sinusoidal wave is in the form of a triangular wave, since the current is set to a lower resonance frequency than that during an ON period of the switch element Q1. The latter current is a resonance current in the form of a sinusoidal wave in which a resonance element of the reactor Lr and the current resonance capacitor C2 appears.
When the switch element Q1 is turned off, energy of the excitation current stored in the transformer T produces voltage pseudo-resonance by (the reactor Lr plus the excitation inductance Lp) and the current resonance capacitor C2, and a voltage resonance capacitor Cry (not illustrated) on both ends of the switch element Q2. At this time, a resonance frequency generated by the voltage resonance capacitor Cry having a small capacitance is observed as a voltage across the switch element Q1 and the switch element Q2. In other words, the current of the switch element Q1 shifts to the voltage resonance capacitor Cry as soon as the switch element Q1 is turned off. When the voltage resonance capacitor Cry is discharged to zero volt, the current shifts to an internal diode of the switch element Q2. The energy of the excitation current stored in the transformer T charges the current resonance capacitor C2 through the internal diode of the switch element Q2. During this period, the switch element Q2 is turned on thereby to enable the switch element Q2 to become a zero-volt switch.
When the switch element Q2 is turned on, with the current resonance capacitor C2 acting as a power supply, an electric current flows through a route from the current resonance capacitor C2 through the primary winding P, the resonance reactor Lr, and the switch element Q2 to the current resonance capacitor C2. The electric current is a combined current of an excitation current flowing through the excitation inductance Lp of the transformer T, and a load current supplied from the output terminal OUT to the load through the primary winding P, the secondary winding S1, the diode D1, and the smoothing capacitor C3. The former current is a resonance current in the form of a sinusoidal wave of (the reactor Lr plus the excitation inductance Lp) and the current resonance capacitor C2, and is observed as a current in which a portion of the sinusoidal wave is in the form of a triangular wave, since the current is set to a lower resonance frequency than that during an ON period of the switch element Q2. The latter current is a resonance current in the form of a sinusoidal wave in which a resonance element of the reactor Lr and the current resonance capacitor C2 appears.
When the switch element Q2 is turned off, energy of the excitation current stored in the transformer T produces voltage pseudo-resonance by (the reactor Lr plus the excitation inductance Lp) and the current resonance capacitor C2, and the voltage resonance capacitor Cry. At this time, a resonance frequency generated by the voltage resonance capacitor Cry having a small capacitance is observed as a voltage across the switch element Q1 and the switch element Q2. In other words, the current of the switch element Q2 shifts to the voltage resonance capacitor Cry as soon as the switch element Q2 is turned off. When the voltage resonance capacitor Cry is discharged to the output voltage from the smoothing capacitor C1, the current shifts to an internal diode of the switch element Q1. The energy of the excitation current stored in the transformer T is regenerated to the current resonance capacitor C1 through the internal diode of the switch element Q1. During this period, the switch element Q1 is turned on thereby to enable the switch element Q1 to become a zero-volt switch.
FIG. 2 illustrates waveforms of portions of the conventional current resonance type power supply device under light load. In FIG. 2, Id(Q1) represents a drain current of the switch element Q1; I(P), a current flowing through the primary winding P; V(C2), a voltage across ends of the current resonance capacitor C2; Vds(Q2), a drain-source voltage of the switch element Q2; V(P), a voltage across ends of the primary winding P; V(D1), a voltage across ends of the diode D1; and V(D2), a voltage across ends of the diode D2.
Also, in the conventional current resonance type power supply device, the switch element Q1 and the switch element Q2 are repeatedly alternately turned on and off at a duty ratio of 50% to control a switching frequency and thereby control an output voltage. At this time, as illustrated in FIG. 2, the voltage V(C2) of the current resonance capacitor C2 repeats charging and discharging symmetrically about ½ of a voltage across ends of a voltage V(C1) of the smoothing capacitor C1. Thereby, the voltage V(P) is generated in the primary winding P, a voltage is generated in the secondary windings S1, S2, and the voltage is rectified by the diodes D1, D2 thereby to obtain an output voltage.
Incidentally, a current resonance type power supply device described for example in Japanese Patent Application Publication No. 2013-78228 and Japanese Unexamined Patent Application Publication No. Hei 7-135769 is known as the related art of the prior art.
However, in the conventional current resonance type power supply device, when an excitation current of the transformer T capable of providing a necessary supply of power to a secondary side under heavy-load conditions is set, the excitation current of the transformer T, even under light-load conditions, flows as a large current and does not become zero, as illustrated in FIG. 2. Also, under the light-load conditions, the switching frequency becomes high, and consequently, even if the excitation current of the transformer T is reduced as compared to that under the heavy-load conditions, a loss of the current resonance type power supply device taken as a whole is not greatly reduced. Thus, efficiency is reduced.
An object of the present invention is to provide a current resonance type power supply device which improves efficiency by reducing an excitation current of a transformer, or equivalently, a charging/discharging current and loss of a current resonance capacitor, under light-load conditions.